Method and circuitry for multi-stage amplification

ABSTRACT

In an amplifier, a first stage receives a differential input voltage, which is formed by first and second input voltages, and outputs a first differential current in response thereto on first and second lines having respective first and second line voltages. A second stage receives the first and second line voltages and outputs a second differential current in response thereto on third and fourth lines having respective third and fourth line voltages. A transformer includes first and second coils. A first terminal of the first coil is coupled through a first resistor to the first line. A second terminal of the first coil is coupled through a second resistor to the second line. A first terminal of the second coil is coupled through a third resistor to the third line. A second terminal of the second coil is coupled through a fourth resistor to the fourth line.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 61/884,444, filed Sep. 30, 2013, entitled METHOD AND CIRCUITRYFOR MULTI-STAGE AMPLIFICATION, naming Swaminathan Sankaran et al. asinventors, which is hereby fully incorporated herein by reference forall purposes.

BACKGROUND

The disclosures herein relate in general to electronic circuitry, and inparticular to a method and circuitry for multi-stage amplification.

FIG. 1 (prior art) is a schematic electrical circuit diagram of aconventional multi-stage amplifier, indicated generally at 100. Theamplifier 100 includes at least first and second stages 102 and 104,which are transconductance amplifiers whose gains are Gm1 and Gm2,respectively. The amplifier 100 receives a differential input voltagefrom lines S₀₀ and S₀₁.

The first stage 102 applies the gain Gm1 to amplify a difference(“ΔN_(IN)”) between S₀₀'s voltage (“V_(IN)+”) and S₀₁'s voltage(“V_(IN)−”). Similarly, the second stage 104 applies the gain Gm2 toamplify a difference (“ΔV₁”) between a line S₁₀'s voltage (“V₁₀”) and aline S₁₁'s voltage (“V₁₁”). Accordingly: (a) in response to ΔV_(IN), thefirst stage 102 generates a difference (“ΔI₁”) between S₀₁'s current(“I₁₀”) and S₁₁'s current (“I₁₁”); and (b) in response to ΔV₁, thesecond stage 104 generates a difference (“ΔI₂”) between a line S₂₀'scurrent (“I₂₀”) and a line S₂₁'s current (“I₂₁”).

As shown in FIG. 1, S₁₀ is connected to a resistor R₁₀, which is coupledthrough a first terminal of an inductor L₁ to a voltage supply nodeV_(DD). Also, S₁₁ is connected to a resistor R₁₁, which is coupledthrough a second terminal of L₁ to V_(DD). Similarly, S₂₀ is connectedto a resistor R₂₀, which is coupled through a first terminal of aninductor L₂ to V_(DD). Further, S₂₁ is connected to a resistor R₂₁,which is coupled through a second terminal of L₂ to V_(DD).

FIG. 2 (prior art) is a graph of an example curve 202 of gain (dB)versus frequency for the amplifier 100, having a 3 dB bandwidth region204. As shown in FIG. 2, the 3 dB bandwidth region 204 is a range offrequencies whose gains are within 3 dB of peak gain. Without L₁, L₂,R₁₀, R₁₁, R₂₀ and R₂₁, performance the amplifier 100 could diminish,according to an example curve 206 having a 3 dB bandwidth region 208.

By comparison, with L₁, L₂, R₁₀, R₁₁, R₂₀ and R₂₁: (a) in the region208, a dominant contribution to the 3 dB bandwidth region 204 isprovided by R₁₀, R₁₁, R₂₀ and R₂₁; and (b) in a bandwidth expansionregion 210, a significant contribution to the 3 dB bandwidth region 204is provided by L₁ and L₂, in addition to contribution by R₁₀, R₁₁, R₂₀and R₂₁.

Nevertheless, the amplifier 100 has shortcomings. For example, theamplifier 100 has one passive magnetic component (e.g., inductor) perstage. As a precaution against possible interference through magneticfield coupling (e.g., between coil windings of nearby inductors), aspacing is imposed between those passive magnetic components, whichincreases silicon area in an integrated circuit that contains theamplifier 100.

SUMMARY

In an amplifier, a first stage receives a differential input voltage,which is formed by first and second input voltages, and outputs a firstdifferential current in response thereto on first and second lineshaving respective first and second line voltages. A second stagereceives the first and second line voltages and outputs a seconddifferential current in response thereto on third and fourth lineshaving respective third and fourth line voltages. A transformer includesfirst and second coils. A first terminal of the first coil is coupledthrough a first resistor to the first line. A second terminal of thefirst coil is coupled through a second resistor to the second line. Afirst terminal of the second coil is coupled through a third resistor tothe third line. A second terminal of the second coil is coupled througha fourth resistor to the fourth line.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) is a schematic electrical circuit diagram of aconventional multi-stage amplifier.

FIG. 2 (prior art) is a graph of an example curve of gain (dB) versusfrequency for the amplifier of FIG. 1.

FIG. 3 is a schematic electrical circuit diagram of a multi-stageamplifier of the illustrative embodiments.

DETAILED DESCRIPTION

FIG. 3 is a schematic electrical circuit diagram of a multi-stageamplifier, indicated generally at 300, of the illustrative embodiments.The amplifier 300 includes at least the first and second stages 102 and104, and the resistors R₁₀, R₁₁, R₂₀ and R₂₁. However, instead of L₁ andL₂ (FIG. 1), the amplifier 300 includes a transformer, indicated bydashed enclosure 302.

The first and second stages 102 and 104 are transconductance amplifierswhose gains are Gm1 and Gm2, respectively. The first stage 102 appliesthe gain Gm1 to amplify the difference (“ΔN_(IN)”) between V_(IN)+ andV_(IN)−. Similarly, the second stage 104 applies the gain Gm2 to amplifythe difference (“ΔV₁”) between S₁₀'s voltage (“V₁₀”) and S₁₁'s voltage(“V₁₁”). Accordingly: (a) in response to ΔV_(IN), the first stage 102generates the difference (“ΔI₁”) between S₁₀'s current (“I₁₀”) and S₁₁'scurrent (“I₁₁”); and (b) in response to ΔV₁, the second stage 104generates the difference (“ΔI₂”) between S₂₀'s current (“I₂₀”) and S₂₁'scurrent (“I₂₁”).

In this example: (a) if ΔV_(IN) is positive, then ΔI₁ is negative; and(b) conversely, if ΔV_(IN) is negative, then ΔI₁ is positive. Similarly,in this example: (a) if ΔV₁ is positive, then ΔI₂ is negative; and (b)conversely, if ΔV₁ is negative, then ΔI₂ is positive.

The transformer 302 includes first and second coils 304 and 306. Asshown in FIG. 3: (a) R₁₀ is coupled through a first terminal of the coil304 to V_(DD); (b) R₁₁ is coupled through a second terminal of the coil304 to V_(DD) (which is coupled to a third terminal of the coil 304);(c) R₂₁ is coupled through a first terminal of the coil 306 to V_(DD);and (d) R₂₀ is coupled through a second terminal of the coil 306 toV_(DD) (which is coupled to a third terminal of the coil 306). Currentflows: (a) through the coil 304 in a first direction; and (b) throughthe coil 306 in a second direction that is substantially identical to(e.g., same as) the first direction.

If the transformer 302 is ideal, lossless and perfectly coupled, thenV₂=n·V₁, I₂=I₁/n, and P_(IN)=P_(OUT), where: (a) V₁ is a voltage acrossthe first and second terminals of the coil 304; (b) V₂ is a voltageacross the first and second terminals of the coil 306; (c) I₁ is acurrent through the coil 304; (d) I₂ is a current through the coil 306;(e) n is a winding turns ratio between the coils 304 and 306, so that nequals winding turns of the coil 306 divided by winding turns of thecoil 304; (f) V₁·I₁=P_(IN), which is input power of the transformer 302;and (g) V₂·I₂=P_(OUT), which is output power of the transformer 302.

The coil 304 provides passive impedance boost to the output lines S₁₀and S₁₁ of the stage 102. Similarly, the coil 306 provides passiveimpedance boost to the output lines S₂₀ and S₂₁ of the stage 104. Also,due to coupling between the coils 304 and 306: (a) the coil 304 providesactive feedback to the output lines S₂₀ and S₂₁ of the stage 104; and(b) the coil 306 provides active feedback to the output lines S₁₀ andS₁₁ of the stage 102. Such active feedback reduces cost and size of thetransformer 302.

Moreover, the stages 102 and 104 can be spaced more closely to oneanother, which reduces silicon area in an integrated circuit thatcontains the amplifier 300. For example, the amplifier 300 includes onetransformer (e.g., the transformer 302) per two stages (e.g., the stages102 and 104), instead of one passive magnetic component per stage. Also,instead of avoiding magnetic field coupling, the transformer 302contains a magnetic field with controlled H-field coupling between thecoils 304 and 306. Such containment reduces proliferation (adulteration)between nearby circuitry (e.g., nearby magnetic devices).

For these various reasons, 3 dB bandwidth of the amplifier 300 isexpanded. For this purpose of bandwidth expansion, the quality factor(“QF”) of the transformer 302 is not required to be high. Accordingly,the amplifier 300 can have a reduced form factor.

Although illustrative embodiments have been shown and described by wayof example, a wide range of alternative embodiments is possible withinthe scope of the foregoing disclosure.

What is claimed is:
 1. An amplifier, comprising: a first stage forreceiving a differential input voltage, which is formed by first andsecond input voltages, and outputting a first differential current inresponse thereto on first and second lines having respective first andsecond line voltages; a second stage coupled to the first stage forreceiving the first and second line voltages and outputting a seconddifferential current in response thereto on third and fourth lineshaving respective third and fourth line voltages; and a transformercoupled to the first and second stages, wherein the transformer includesfirst and second coils, wherein a first terminal of the first coil iscoupled through a first resistor to the first line, wherein a secondterminal of the first coil is coupled through a second resistor to thesecond line, wherein a first terminal of the second coil is coupledthrough a third resistor to the third line, and wherein a secondterminal of the second coil is coupled through a fourth resistor to thefourth line.
 2. The amplifier of claim 1, wherein a voltage supply nodeis coupled to: a third terminal of the first coil; and a third terminalof the second coil.
 3. The amplifier of claim 1, wherein the first andsecond stages are first and second transconductance amplifiers havingfirst and second gains, respectively.
 4. The amplifier of claim 1,wherein the first coil is for conducting current in a first direction,and wherein the second coil is for conducting current in a seconddirection that is substantially identical to the first direction.
 5. Theamplifier of claim 1, wherein the first coil is for providing passiveimpedance boost to the first and second lines, and wherein the secondcoil is for providing passive impedance boost to the third and fourthlines.
 6. The amplifier of claim 1, wherein the first coil is forproviding active feedback to the third and fourth lines, and wherein thesecond coil is for providing active feedback to the first and secondlines.
 7. The amplifier of claim 1, wherein the transformer is forcontaining a magnetic field with controlled H-field coupling between thefirst and second coils.
 8. An amplifier, comprising: a first stage forreceiving a differential input voltage, which is formed by first andsecond input voltages, and outputting a first differential current inresponse thereto on first and second lines having respective first andsecond line voltages, wherein the first stage is a firsttransconductance amplifier having a first gain; a second stage coupledto the first stage for receiving the first and second line voltages andoutputting a second differential current in response thereto on thirdand fourth lines having respective third and fourth line voltages,wherein the second stage is a second transconductance amplifier having asecond gain; and a transformer coupled to the first and second stages,wherein the transformer includes first and second coils, wherein a firstterminal of the first coil is coupled through a first resistor to thefirst line, wherein a second terminal of the first coil is coupledthrough a second resistor to the second line, wherein a first terminalof the second coil is coupled through a third resistor to the thirdline, wherein a second terminal of the second coil is coupled through afourth resistor to the fourth line, wherein the first coil is forconducting current in a first direction, and wherein the second coil isfor conducting current in a second direction that is substantiallyidentical to the first direction; wherein a voltage supply node iscoupled to: a third terminal of the first coil; and a third terminal ofthe second coil.
 9. The amplifier of claim 8, wherein the first coil isfor providing passive impedance boost to the first and second lines andproviding active feedback to the third and fourth lines, and wherein thesecond coil is for providing passive impedance boost to the third andfourth lines and providing active feedback to the first and secondlines.
 10. The amplifier of claim 9, wherein the transformer is forcontaining a magnetic field with controlled H-field coupling between thefirst and second coils.
 11. A method, comprising: with a first stage ofan amplifier, receiving a differential input voltage, which is formed byfirst and second input voltages, and outputting a first differentialcurrent in response thereto on first and second lines having respectivefirst and second line voltages; with a second stage of the amplifier,receiving the first and second line voltages and outputting a seconddifferential current in response thereto on third and fourth lineshaving respective third and fourth line voltages; and coupling: a firstterminal of a first coil of a transformer through a first resistor tothe first line; a second terminal of the first coil through a secondresistor to the second line; a first terminal of a second coil of thetransformer through a third resistor to the third line; and a secondterminal of the second coil through a fourth resistor to the fourthline.
 12. The method of claim 11, wherein the coupling includes:coupling a voltage supply node to: a third terminal of the first coil;and a third terminal of the second coil.
 13. The method of claim 11,wherein the first and second stages are first and secondtransconductance amplifiers having first and second gains, respectively.14. The method of claim 11, and comprising: through the first coil,conducting current in a first direction; and through the second coil,conducting current in a second direction that is substantially identicalto the first direction.
 15. The method of claim 11, and comprising: withthe first coil, providing passive impedance boost to the first andsecond lines; and with the second coil, providing passive impedanceboost to the third and fourth lines.
 16. The method of claim 11, andcomprising: with the first coil, providing active feedback to the thirdand fourth lines; and with the second coil, providing active feedback tothe first and second lines.
 17. The method of claim 11, and comprising:with the transformer, containing a magnetic field with controlledH-field coupling between the first and second coils.
 18. A method,comprising: with a first stage of an amplifier, receiving a differentialinput voltage, which is formed by first and second input voltages, andoutputting a first differential current in response thereto on first andsecond lines having respective first and second line voltages, whereinthe first stage is a first transconductance amplifier having a firstgain; with a second stage of the amplifier, receiving the first andsecond line voltages and outputting a second differential current inresponse thereto on third and fourth lines having respective third andfourth line voltages, wherein the second stage is a secondtransconductance amplifier having a second gain; coupling: a firstterminal of a first coil of a transformer through a first resistor tothe first line; a second terminal of the first coil through a secondresistor to the second line; a first terminal of a second coil of thetransformer through a third resistor to the third line; and a secondterminal of the second coil through a fourth resistor to the fourthline; through the first coil, conducting current in a first direction;through the second coil, conducting current in a second direction thatis substantially identical to the first direction; and coupling avoltage supply node to: a third terminal of the first coil; and a thirdterminal of the second coil.
 19. The method of claim 18, and comprising:with the first coil, providing passive impedance boost to the first andsecond lines, and providing active feedback to the third and fourthlines; and with the second coil, providing passive impedance boost tothe third and fourth lines, and providing active feedback to the firstand second lines.
 20. The method of claim 19, and comprising: with thetransformer, containing a magnetic field with controlled H-fieldcoupling between the first and second coils.